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Geology : Paleontology
ROCKS AND LAYERS
We study Earth's history by
studying the record of past events that is preserved in the rocks. The layers of
the rocks are the pages in our history book.
As early as the
mid-1600's, the Danish scientist Nicholas Steno studied the relative positions
of sedimentary rocks. He found that solid particles settle from a fluid
according to their relative weight or size. The largest, or heaviest, settle
first, and the smallest, or lightest, settle last. Slight changes in particle
size or composition result in the formation of layers, also called beds, in the
rock. Layering, or bedding, is the most obvious feature of sedimentary rocks.
Sedimentary rocks are formed particle by particle and bed by bed, and the
layers are piled one on top of the other. Thus, in any sequence of layered
rocks, a given bed must be older than any bed on top of it. This Law of
Superposition is fundamental to the interpretation of Earth history,
because at any one location it indicates the relative ages of rock layers and
the fossils in them.
Fossils, Rocks, and Time
By Lucy E. Edwards and John Pojeta, Jr.
Contents & Introduction
Putting Events in Order
The Relative Time Scale
Rocks and Layers
Fossils and Rocks
The Numeric Time Scale